System for homogeneously mixing plural incoming product streams of different composition

ABSTRACT

A continuous mixer ( 32 ) is disclosed which can be used for mixing of incoming product streams ( 22, 24 ) of different characteristics respectively to yield a final product stream ( 26 ) of predetermined, consistent characteristics. The mixer ( 32 ) includes an elongated housing ( 42 ) having a pair of product input ports ( 50, 52 ) and an output ( 64 ), with a pair of elongated, axially rotatable, mixing screws ( 44, 46 ) located within the housing ( 42 ). The screws ( 44, 46 ) include a series of outwardly projecting mixing elements ( 114 ) preferably of pyramidal design and arrayed in a helical pattern along the length of the screws ( 44,46 ). The mixer ( 32 ) may be used in a processing system ( 20,200 ) having individual product lines ( 28, 30, 204, 206 ) coupled to the mixer ( 32 ), and is especially useful for processing of incoming meat streams ( 22,24 ) of different fat/lean ratios, to give a final comminuted output stream ( 26 ) of an intermediate and essentially constant fat/lean ratio. Preferably, the product lines ( 204,206 ) are each equipped with a combine preblender and pumping device ( 202 ).

RELATED APPLICATION

[0001] This is a division of Ser. No. 09/781,719, filed Feb. 2, 2001,which is a continuation-in-part of application Ser. No. 09/740,448 filedDec. 19, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is broadly concerned with continuous mixingapparatus for the gentle yet thorough mixing of incoming product streamsto yield a final product stream of predetermined desiredcharacteristics. More particularly, the invention is concerned with suchmixers, rotatable screws used therein, and methods of operation thereof,permitting the mixers to be used in overall systems preferably designedfor the mixing of dissimilar characteristic incoming meat streams toproduce a final product output stream of substantially constantcharacteristics. In addition, the mixer may also be used in theprocessing of substantially homogeneous products, such as in the mixingand stretching of cheese curd and the blending of fruits.

[0004] 2. Description of the Prior Art

[0005] The meat industry is increasingly concerned with “case ready”meats which are prepared and packaged at a central processing facility,ready for display and sale at supermarkets. This marketing approachminimizes costly on-site labor at the supermarkets, leading to lowerconsumer prices. For example, comminuted meat products (e.g., hamburger)can be produced at a central facility and packaged in convenient sizedconsumer packages. One difficulty in this approach, however, lies inproviding a consistent comminuted product having, e.g., the samefat/lean ratio. This is particularly difficult owing to the fact thatstarting meat sources may have very different fat/lean ratios, on aday-to-day or even hour-to-hour basis. Hence, a plant may be providedwith meats having two widely divergent meat sources in terms of fat/leanratios or other characteristics, and must be capable of accommodatingsuch staring materials while still producing a comminuted product ofsubstantially constant final properties.

[0006] Meat comminuting and mixing devices are in general well known,ranging from simple household sausage grinders to large industrialequipment. However, such prior devices cannot properly handle diversestarting products while still yielding consistent final products. Inaddition, it is important in the mixing and handling of meat productsthat the meat not be comminuted and mixed to the point that it exhibits“smearing” or the loss of particulate appearance.

[0007] There is accordingly a need in the art for improved mixingapparatus and systems which can accept starting products of divergentand changing characteristics while nevertheless producing final productshaving predetermined, consistent properties; in the context of meatprocessing, such mixing apparatus must also accomplish these endswithout significantly altering the desired meat appearance.

SUMMARY OF THE INVENTION

[0008] The present invention overcomes the problems outlined above, andprovides a continuous mixer for mixing incoming product streams to yielda final product stream of desired characteristics. Broadly speaking, themixer includes an elongated housing having a plurality of inputs forreceiving incoming product streams, as well as an output for the finalproduct. A plurality (usually two) of elongated, side-by-side, axiallyrotatable mixing screws are located within the housing and extend alongthe length thereof in order to convey and mix the incoming streams andto move the final product out the housing output. Each of these mixingscrews includes a series of outwardly projecting mixing elements eachhaving a base and a plurality of converging sidewall surfaces, thelatter cooperatively defining an outer end having a surface area lessthan the base surface area. These mixing elements are oriented along thelength of the mixing screws, preferably in a helical pattern.

[0009] In preferred forms, the mixing screws are in intermeshingrelationship and are designed to co-rotate, i.e., to rotate in the samedirection; however, the screws can also be counter-rotating if desired.The screws preferably include input sections adjacent the housingproduct stream inputs and present helical flighting along the lengthsthereof; the screws also have output sections extending from the ends ofthe input sections toward the housing output, with the outwardlyprojecting mixing elements being located on the output sections. Theindividual mixing elements are generally pyramidal in shape, presentinga base of generally diamond-shaped plan configuration with fouroutwardly extending, arcuate converging wall surfaces terminating in anapex-like outer end.

[0010] The housing is equipped with a plurality of injection portsspaced along the length thereof to permit injection of materials such asCO₂ into the housing during operation. In addition, the housing also aseries of sensor ports along the length thereof to permit installationof temperature, pressure or other parameter sensors. In order to providebetter temperature control, the housing has an outer shell and innerscrew-receiving walls to define therebetween a passageway; cold water orother cooling media may be circulated through the passageway duringoperation of the mixer.

[0011] A particular (although not exclusive) utility of the mixer of theinvention is for producing a comminuted meat product having apredetermined and substantially constant fat/lean ratio, using inputmeat streams of different fat/lean ratios respectively. To this end, themixer is preferably used in an overall mixing system including aplurality of incoming product lines operably coupled with the mixer,where each of the product lines has a product source, a product pump anda product analyzer. In such a system, the pumps are operated to generatethe incoming product streams, which are analyzed to determine a desiredinput characteristic thereof (such as fat/lean ratio). The operationalspeed of the individual product line pumps is then adjusted in responseto analysis of the incoming product streams, thereby generating productstreams having a desired input characteristic at a substantiallyconstant magnitude for each incoming product stream. Once such constantcharacteristic streams are achieved the incoming product streams aredirected to the mixer which is operated to create the final productstream. Preferably, this final product stream was again analyzed todetermine a desired output characteristic thereof, followed by alteringthe operational speed of one or more of the product line pumps asnecessary to maintain the desired output characteristic in the finalproduct stream.

[0012] In another embodiment, each of the incoming product linesincludes a combined preblender and pumping device in lieu of separatepreblenders and pumps. Such a combined device preferably includes anupper hopper equipped with a pair of rotatable shafts having outwardlyextending paddle elements. Also, the device includes a lower pumpingsection made up of side-by-side, fully intermeshed, rotatable augerscrews which create the necessary pumping force to convey product fromthe combined device for downstream processing.

[0013] The preferred mixer is designed so as to mix incoming productstreams and create a homogeneous output of substantially constantcharacteristics, without undue meat comminution or smear. In practice,the mixers of the invention are operated so as to limit meat temperatureto no greater than about 50° F., more preferably from about 20-40° F.Residence time in the mixers of the invention should range up to about 3minutes, more preferably from about 1-2 minutes; pressure conditionswithin the mixer are essentially atmospheric, but the mixer may beoperated at a slight positive pressure if desired.

[0014] While the system and continuous mixer of the invention areespecially adapted for use in the meat industry, a number of variationsare possible. For example, spices or liquid smoke may be injected intothe continuous mixer to produce sausage-like products. Alternately,textured vegetable protein may be added to one or more of the meatstreams, or the system can be used to mix a meat stream and a TVPstream, respectively. Finally, the mixer of the invention, owing to itsunique screw configuration, may be used for the processing of non-meatproducts such as cheeses, fruits and vegetables.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic flow diagram illustrating the preferredsystem of the invention for homogeneously nixing a plurality of incomingproduct streams to yield a final product stream of desiredcharacteristics;

[0016]FIG. 2 is a flow diagram of the preferred software algorithm usedin the control of the system illustrated in FIG. 1;

[0017]FIG. 3 is an isometric view of the preferred continuous mixerforming a part of the system of FIG. 1;

[0018]FIG. 4 is an isometric view similar to that of FIG. 3, but withcertain parts broken away to reveal the internal construction of themixer;

[0019]FIG. 5 is a plan view of the mixer depicted in FIG. 3;

[0020]FIG. 6 is an end view of the mixer shown in FIG. 5, illustratingthe output end of the mixer;

[0021]FIG. 7 is a sectional view taken along line 7-7 of FIG. 6;

[0022]FIG. 8 is a sectional view taken along line 8-8 of FIG. 5;

[0023]FIG. 9 is a plan view of a mixer screw section, depicting thegenerally pyramidal mixing elements forming a part of the preferredinternal mixing screws of the continuous mixer;

[0024]FIG. 10 is an isometric view of the mixing screw sectionillustrated in FIG. 9.

[0025]FIG. 11 is an isometric view of a combined preblender and pumpapparatus useful in the systems of the invention;

[0026]FIG. 12 is an elevational view of the apparatus depicted in FIG.11;

[0027]FIG. 13 is a plan view of the FIG. 11 apparatus;

[0028]FIG. 14 is a front end view of the FIG. 11 apparatus;

[0029]FIG. 15 is a vertical sectional view taken along line 15-15 ofFIG. 14 and illustrating the internal construction of the combinedapparatus;

[0030]FIG. 16 is a horizontal sectional view taken along line 16-16 ofFIG. 12;

[0031]FIG. 17 is a vertical sectional view taken along line 17-17 ofFIG. 12; and

[0032]FIG. 18 is a schematic representation of the continuous mixer ofthe invention, with a pair of the combined preblender and pump devicescoupled thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Turning now to the drawings, and particularly FIG. 1, a preferredsystem 20 is schematically illustrated for homogeneously mixing pluralincoming product streams 22 and 24 to yield a desired final productstream 26. Broadly speaking, the system 20 includes a pair of incomingproduct lines 28, 30 which are operatively coupled with a continuousmixer 32. The system 20 as shown is designed for processing first andsecond meat sources M1 and M2 having different fat/lean ratios in orderto generate the final product stream 26 having a desired andpredetermined fat/lean ratio.

[0034] In more detail, each of the product lines 28, 30 includes agrinder 33 and a preblender 34, a product pump 36, and a fat contentanalyzer 38. The grinder/preblend equipment 33, 34 is essentiallyconventional and is designed to take an incoming meat source andgenerate a uniform ground meat output. Similarly, the pump 36 andanalyzer 38 are conventional.

[0035] As also illustrated in FIG. 1, a fat content analyzer 40 is usedto determine the fat content of the final product stream 26; to thisend, the analyzer 40 is downstream of mixer 32 and upstream of packagingequipment (not shown) used to package the final product. The output fromanalyzer 38 includes a three-way valve 39 with a recycle line 39 a and amixer conduit 39 b.

[0036] Attention is next directed to FIGS. 3-10 which illustrate indetail the preferred mixer 32. Broadly, the mixer 32 includes anelongated housing 42 with a pair of elongated, side-by-side, axiallyrotatable, intermeshed mixing screws 44, 46 located within the housingand extending along the length thereof; the screws 44, 46 are rotated bymeans of a conventional gear reduction drive 48 coupled to a motor (notshown).

[0037] The housing 42 includes an inlet head 49 having a pair ofopposed, tubular inlet ports 50, 52 and end walls 54, 56, as well as anoutlet head of 60 presenting an end wall 62 and an elongated slot-likemixer output 64. As shown, the housing 42 also has two aligned headsections 66 and 68 between the inlet and outlet heads 49 and 60. Thesection 66 has a pair of circular end walls 70, 72; an outermost,elongated circular in cross-section shell wall 74 as well as anelongated, inner, screw-receiving wall 76 of somewhat “figure 8”configuration extend between and are supported by the walls 70, 72.Similarly, the section 68 has end walls 78, 80 supporting shell wall 82and inner screw-receiving “figure 8” wall 84. As illustrated in FIG. 3for example, the circular walls 72 and 78 are bolted together, with endwall 80 connected to reducer 48 and with end wall 70 coupled with wall62 through an intermediate annular spacer 86; in this way, a housing 42is provided with continuous inner screw-receiving walls.

[0038] The housing head sections 66 and 68 are each equipped with aseries of injection ports 88 along the length thereof which permitattachment of injectors (not shown) for the selective injection ofadditives and/or coolants such as carbon dioxide. As best illustrated inFIG. 7, the ports 88 extend through the outer and inner housing walls tocommunicate with the interior of the housing. Also, the sections 66 and68 have sensor mounts 90 along the length thereof for selective mountingand attachment of temperature or other type of sensors. In the use ofmixer 32 as more fully described below, a liquid coolant may be passedthrough respective coolant passageways 92 and 94 provided between theouter and inner housing walls 74, 76 and 82, 84; to this end, the headsections 66 and 68 have appropriately sized and configured coolant entryports 96 and corresponding outlet ports (not shown).

[0039] The screws 44, 46 are housed within and extend along the lengthof the housing 42. As shown in FIG. 8, the screws are positioned withinthe “figure 8” housing walls 76 and 84, and are operatively coupled tothe drive 48 for rotation thereof. The screws include a respectiveelongated splined shafts 98, 100, which support corresponding inletscrews 102, 104 and downstream mixing screws 106, 108. The inlet screwseach include continuous helical double flighting 110, 112 which servesto move material entering the mixer through the inlets 50, 52 towardoutlet head 60.

[0040] The mixing screws 106, 108 are secured to the shafts 98, 100 andare of specialized configuration to mix the incoming products andproduce a uniform output, without creation of undue shear conditions.Attention is directed to FIGS. 9 and 10 which depict in detail thepreferred configuration of the mixing screws. Specifically, each of themixing screws has a series of outwardly projecting, abutting mixingelements 114, each presenting a base 116 and a plurality of convergingsidewall surfaces 118, 120, 122, and 124 which terminate in an outersurface 126 having a surface area less than that of the base 116. Theelements 114 are oriented in a dual helix pattern along the length ofthe screw section, leaving corresponding helical base surfaces 128 and129 between the convolutions of the elements 114.

[0041] In more detail, each of the elements 114 is generally pyramidalin shape, with the corresponding base 116 generally diamond-shaped inplan configuration and presenting four arcuate surfaces 118-124 and theapex-like surface 126. Each diamond-shaped base 116 is defined by twopairs of substantially parallel marginal base surfaces, namely longsurfaces 130, 132 and short surfaces 134, 136. As best seen in FIG. 9,the long base surfaces 130 of the elements 114 lie along a first helicalline 138 whereas the opposed long base surfaces 132 lie along a secondhelical line 140, with the helical lines 138, 140 being of substantiallyequal pitch. In addition, the short base surfaces 134 cooperativelydefine a third helical line 142, with the opposed short base surfaces136 defining a fourth helical line 144. Again, the helical lines 142,144 are parallel to each other, and have substantially the same pitch.However, the pitch of the long base surface helical lines 138, 140 isgreater than the pitch of the short base surface helical lines 142 and144. It will also be seen that the outwardly extending surfaces of theelements 114 lie in and cooperatively define respective helicalsurfaces.

[0042] The mixing screws 106, 108 are preferably manufactured by firstcreating a screw with conventional double helix flighting having thelarger pitch referred to previously. Thereafter, this screw is cut topresent double helix reverse flighting having the smaller pitchmentioned above. This manufacturing procedure creates the series ofmixing elements 114.

[0043] In the operation of mixer 32, incoming products are directedthrough the ports 50, 52 into the interior of the housing 42. At thesame time, the screws 44, 46 are rotated so as to move the productstowards outlet opening 64. During traversal of the inlet sections 102and 104, only a minor amount of mixing occurs. However, as the productsenter and pass along the length of the mixing screws 106, 108, theproduct is very intensely mixed so as to yield a final product stream 26of uniform characteristics. A significant advantage of the mixer 32 isthat such product stream mixing is obtained without substantial heatingof the products or generation of shear. This effect is achieved by thegeometry of the helically arranged mixing elements 114 which serve tonot only move the product toward the outlet 64, but also impart asignificant amount of flow reversal to the products. Of course, the netmovement of the products within the housing is from the inlet ports tothe outlet; nevertheless, during such movement there is significant flowreversal so as to obtain the desired homogeneous final product stream.

[0044] During the course of mixing, it may be desirable to pass thermalfluid (e.g., cold water or a heating media to inhibit fat buildup)through the passageways 92 and 94 so as to indirectly cool the products.Also, carbon dioxide may be injected through some or all of the ports 88for this purpose. Process control is facilitated by means of the mounts90, allowing temperature probes or the like to be mounted along thelength of the mixer.

[0045] In preferred forms, the system 20 is designed for creating anoutput stream 26 of predetermined and substantially constant fat/leanratio, using two individual meat sources M1 and M2 of different fat/leanratios. Moreover, the system 20 is advantageously configured forcomputer control. That is, the components of the system 20, includingthe grind/preblend devices 33, 34, pumps 36, mixer 32 and fat contentanalyzers 38 and 40 are appropriately connected to a microprocessor (notshown). Additionally, all of the components of the system 20 are usuallyprovided with CO₂ injection apparatus so as to maintain, to the extentfeasible, the meat being processed under oxygen-free or at leastoxygen-minimized conditions.

[0046]FIG. 2 depicts a suitable control program useful in the context ofthe invention. In particular, in the first steps 146 and 148, an initialspeed for the pumps 36 is calculated. This involves inputting into thecontrol program fat/lean ratio estimates for the respective meatstreams, the desired fat/lean ratio of the output stream 26, and thedesired final output rate. Also, fat and lean meat densities values, aswell as pump volumes per cycle of the pumps 36 is retrieved from thecomputer memory. This information is used step 148 to calculate theinitial pump speed for each of the pumps 36.

[0047] In the next step 150, the program initiates operation of thegrinders and preblenders 33, 34., and also begins the operation of thepumps 36 at the initially calculated speed (step 151). After anappropriate data acquisition delay (e.g., 10 seconds), the fat contentin the respective streams is measured in step 152. Such measurements aretaken repeatedly, and the measured fat data obtained during eachmeasurement cycle are stored in computer memory.

[0048] The program next determines in step 154 whether the fat/leanratios of the respective streams are within predetermined limits, suchas ±2%. If this stability has not been achieved, then the programproceeds to step 156 wherein the valves 39 are diverted to recycle therespective meat streams back to the preblenders 34 via lines 39 a, andthe process of steps 152-156 is repeated, using the newly calculatedpump speeds.. Once the stability requirement of step 154 is met, theprogram proceeds to step 158 where the operation of the mixer 32 iscommenced at a preset speed correlated with the desired final outputrate. Mixer operation may also involve circulation of a thermal fluidthrough the ports 96 and the corresponding outlets, and/or injection ofcoolant or other additives through the injection ports 88. Also, in step160, both meat streams are diverted to mixer 32 by appropriate operationof the valves 39.

[0049] In step 162, the previously measured and stored fat content datafor the meat products at and about to enter the continuous mixer inletsis retrieved, and the pump speeds are recalculated; as necessary, thesepump speeds are changed in step 164.

[0050] As mixed product emerges from mixer outlet 64, the fat contentthereof is measured in analyzer 40, as set forth in step 164. Thispermits a calculation (step 166) of which line 22 or 24 would benefitmost from correction of pump speed. That is, it is desirable to operatethe pumps 136 as close as possible to the middle of the operating rangeof the pumps. Accordingly, in step 168, appropriate correction factorsare used to adjust the speed of the pumps 36. In this way, stablerunning conditions can be achieved and maintained throughout the courseof a given run. By the same token, if the characteristics of either orboth of the meat sources M1 and M2 change, this change can beaccommodated within practical limits to maintain consistency in thefinal product.

[0051] Turning to FIG. 18, a system 200 is schematically illustrated forhomogeneously mixing plural incoming product streams of meat or the liketo yield a final output stream of desired characteristics. The system200 is similar in many respects to that illustrated in FIG. 1. However,in this instance, the system is considerably simplified through the useof a pair of combined preblender and pumping devices 202 in lieu of thepreblenders 34, pumps 36 and related equipment. In particular, thesystem 200 again makes use of separate incoming product lines 204, 206which are designed to handle the divergent incoming product streams.Each line 204, 206 includes, for meat handling operations, a grinder 208directly coupled to and feeding the devices 202. The output from thelatter passes through a fat analyzer 210 and thence into the continuousmixer 32. The system downstream of the mixer 32 is substantiallyidentical to that described with reference to system 20 of FIG. 1.

[0052] Broadly speaking, the combined preblender and pump device 202includes an open top hopper preblend section 212 with a lower,twin-screw feeder-pump unit 214. In the embodiment shown, the unit 214is equipped with a transition 216 adapted for coupling directly to theinput of fat analyzer 210.

[0053] The upper preblend section 212 includes an elongated hopper 218defined by arcuate sidewalls 220, 222, forward end 224 and rear end wall226. The sidewalls 220, 222 merge to form a bottom wall 228 whichextends from rear end wall 226 forwardly, but has, adjacent the forwardend thereof, an outlet section 230 made up of vertical end wall 232 andside margins 234, 236. Thus, an outlet opening 237 is defined betweenthe wall 232, margins 234, 236 and front end wall 224. Although notshown, the hopper 218 is provided with a top wall covering the upper endof the hopper in a substantially air tight fashion. As illustrated, thehopper 218 is supported on an upright frame 238 to assume an elevatedposition.

[0054] The hopper 218 is equipped with a pair of elongated, fore and aftextending, axially rotatable mixing shafts 240, 242 which areconventionally powered by a motor (not shown). As best seen in FIG. 13,the shafts 240, 242 are each equipped with a plurality of outwardlyextending, elongated, staggered paddle elements 244. The latter areoriented so as to permit free rotation of the shafts 240, 242; however,at the mid-section of the hopper 218, the elements 244 are intercalated.The respective paddle elements 244 are designed to preblend incomingproduct into the hopper 218, and to move such material forwardly towardsand into outlet section 230. The rear end wall 226 is equipped with atubular product input 246 which is designed to be connected with theoutput of a grinder 208, again in a substantially air tight manner.

[0055] The feeder pump unit 214 includes an elongated housing 248secured to the underside of hopper 218 and extending forwardlytherefrom. The housing 248 has a pair of side-by-side, communicatingarcuate sections 250, 252 with an elongated rearward opening thereinwhich mates with and forms a continuation of outlet opening 237. A pairof elongated, axially flights auger screws 254, 256 are located withinhousing 248 and extend along the length thereof. As best seen in FIG.16, the screws 254, 256 include rearward mixing sections 258, 260designed to mix product from hopper 212, as well as forward pumpingsections 262, 264. The pumping sections are single flight (althoughmultiple flights could be used) fully intermeshed screw sectionsdesigned to create a positive pumping force to propel preblended producttowards the outlet of device 202. Preferably, the screws 254,256 arecounterrotating, but co-rotating designs could also be used. The screws254, 256 are rotated by means of a conventional motor and gear reducerdrive (not shown).

[0056] The forward end of housing 248 is equipped with a transition 266which is in the form of a rectangular block having a converging internalpassageway 268 and a substantially flat output face 270. The purpose oftransition 266 is to direct product pumped via the unit 214 into theinlet of fat analyzer 210, and to also move product through the fatanalyzer and into continuous mixer 32.

[0057] In the use of the devices 202, meat or other product to beprocessed is delivered from the grinder 208 through input 246 anddirectly into hopper 218. If desired, the grinder 208 and the device 202are pressurized with carbon dioxide in order to minimize oxidation ofthe product during processing. Once the product enters hopper 218, it isimmediately subjected to mixing by rotation of the shafts 240, 242 andthe consequent action of paddle elements 244. The product movesforwardly from input 246 and progressively passes into and throughopening 237 whereupon the product enters the housing 248. Additionalmixing is carried out in the rearward section of the housing 248, at thearea of screw sections 258,260. As the product advances along the lengthof these mixing screw sections, it next encounters the pumping screwsections 262, 264. Owing to the fully intermeshed construction of thesections 262, 264, the product is forced along the remaining length ofthe housing 248 and thence into and through transition 266, analyzer 210and into continuous mixer 32. The mixer 32 operates in the same mannerdescribed previously with reference to system 20 in order to generate afinal product stream 26 which is fat-analyzed and packaged.

[0058] The system 200 using the combined preblender and pump devices 202can be controlled using software analogous to that described withreference to FIG. 2. Those skilled in the art will appreciate thatcertain changes would be made in the control software as compared withthat shown in FIG. 2, but these are within the skill of the art.

[0059] A principal advantage obtained through use of the system 200 isthe elimination of separate preblenders and pumps, and the associatedconnection hardware and control elements. Thus, the system 200 can bemore economically produced and operated.

We claim:
 1. A screw set for a mixer, comprising: a pair of elongated,side-by-side, axially rotatable mixing screws each having a centralsection and a series of mixing elements projecting outwardly from saidcentral section, said mixing elements each having a base and a pluralityof converging sidewall surfaces extending outwardly from said base topresent an outer end having a surface area less than the surface area ofthe base, at least certain of said elements being oriented in a helicalpattern along at least a portion of said central section.
 2. The screwset of claim 1, each of said screws having an input section adjacentsaid inputs and presenting helical flighting along the length thereof,and an output section extending from the end of said input section, saidmixing elements being located on said output section.
 3. The screw setof claim 2, the flighting of said input sections, of said screws beingintermeshed, and the mixing elements of the output sections of saidscrews also being intermeshed.
 4. The screw set of claim 1, each of saidmixing elements being generally pyramidal in shape, presenting a basewhich is generally diamond-shaped in plan configuration with fouroutwardly extending and converging sidewall surfaces terminating in aapex-like outer end.
 5. The screw set of claim 2, the diamond-shapedbase having two pairs of substantially parallel base surfaces, one ofsaid base surface pairs having a length greater than the other of saidbase surface pairs.
 6. The screw set of claim 5, the base surfaces ofsaid one surface pairs lying along respective, parallel first helices ofa first, substantially equal pitch, and the base surfaces of said othersurface pairs lying along respective, parallel, second helices of asecond, substantially equal pitch, said second pitch being smaller thansaid first pitch.